This application relates generally to condition sensors and more particularly to sensors such as accelerometers having acceleration responsive means which are movable for providing an electrical signal in response to the existence of acceleration of a body being monitored.
In U.S. Pat. No. 5,345,823, assigned to the assignee of the instant invention, compact, rugged and inexpensive accelerometer devices are disclosed comprising a stiff, rigid, electrically insulating substrate having a recess in the form of a groove formed in one substrate surface. Electrically conductive film means are deposited on that surface to define a capacitor detect plate inside the recess, a capacitor source plate connector pad outside the recess and circuit paths which are connected to the detect plate and source plate connector and to respective terminal pads along an edge of the substrate surface. The accelerometer device includes a flat, electrically conductive plate or blade member of stiffly resilient metal which is formed with an attachment portion, a capacitor source plate portion and integral resilient beam means in a common plane. The attachment portion of the member is secured in electrically conductive relation to the source plate connector on the substrate with a thin layer of solder. Spacing between the upper surfaces of the detect plate and the source plate is determined by the depth of the recess. In one embodiment glass frit, including a bonding glass meltable at one temperature and glass rods of a small, precisely determined diameter which remain shape-retaining at the melting temperature of the bonding glass, is deposited over two spaced portions of the source connector to provide a precise level of attachment of the source plate to the source plate connector.
Although devices made in accordance with the teachings of the above referenced patent are very effective and perform well, the use of solder as a means of attachment involves a relatively time consuming reflow process and requires flux cleansing. In addition, there is a tendency for elements connected by solder to move slightly over time due to so-called creep thereby adversely affecting calibration of the device.
In U.S. Pat. No. 5,239,871, assigned to the assignee of the present invention, another accelerometer device is shown comprising an essentially flat electrically insulating substrate with a detect plate and a source plate connector disposed thereon and with a similar conductive blade member secured to the substrate in electrically conductive relation to the capacitor source plate connector. Spacing between the source plate portion and the detect plate is accomplished by using a shim between the attachment plate portion and source plate connector in one embodiment and in another embodiment by reducing the thickness of a portion of the metal plate. However, use of a shim adds an additional part and process step which adds to the cost of the device. Further, when using a shim it is difficult to obtain close dimensional control from one device to another. Use of a metal blade having a reduced thickness portion results in a more expensive blade member and one for which it is difficult to provide a properly balanced and mounted source plate portion.
In copending application Ser. No. 08/148,042, assigned to the assignee of the present invention, an accelerometer device is shown comprising a substrate having an electrically conductive detect plate on a top surface thereof and a bore between top and bottom surfaces of the substrate. An electrically conductive metal plate or blade having an attachment portion, a source plate portion and an integral beam means connecting the attachment portion to the source plate portion is fixedly attached, as by welding, to the end of an electrically conductive pin inserted in the bore with the source plate portion spaced a selected distance from the detect plate and with the pin in electrical engagement with a conductive path which extends from the top surface of the substrate into the bore. The axial position of the pin is adjusted to provide a selected capacitance level between the detect plate and the source plate portion.
Accelerometer devices made in accordance with the teachings of the copending application are compact and rugged while at the same time have lower thermal errors than prior art devices.